Smile and Say, "Woof!"
Not every pet will want to dress up for Halloween -- some love it, some loathe it. Don't force them, but if they love it take full advantage. Don't bother trying to pose your pets; shooting candid photos of your fur-kids will be more relaxing and fun for you and them. Just be sure to shoot dozens of frames (try turning on the continuous mode setting on your digital camera) so you don't miss that one great shot.
For safety, costumes shouldn't constrict your pet's movements, have any dangling pieces or have any pieces that could be chewed off and swallowed.